The PD-L1/PD-1 pathway promotes dysfunction, but not "exhaustion", in tumor-responding T cells from pleural effusions in lung cancer patients.

Malignant pleural effusions are frequent in patients with advanced stages of lung cancer and are commonly infiltrated by lymphocytes and tumor cells. CD8+ T cells from these effusions have reduced effector functions. The programmed death receptor 1(PD-1)/programmed death ligand 1 (PD-L1) pathway is involved in T-cell exhaustion, and it might be responsible for T-cell dysfunction in lung cancer patients. Here, we show that PD-L1 is expressed on tumor cell samples from malignant effusions, on lung cancer cell lines, and, interestingly, on MRC-5 lung fibroblasts. PD-L1 was up-regulated in lung cancer cell lines upon treatment with IFN-gamma, but not under hypoxic conditions, as detected by RT-qPCR and flow cytometry. Blockade of PD-L1 on tumor cells restored granzyme-B expression in allogenic CD8+ T cells in vitro. Remarkably, pleural effusion CD8+ T cells that responded to the tumor antigens MAGE-3A and WT-1 (identified as CD137+ cells) were lower in frequency than CMV pp65-responding CD8+ T cells and did not have an exhausted phenotype (PD-1+ TIM-3+). Nonetheless, tumor-responding CD8+ T cells had a memory phenotype and expressed higher levels of PD-1. A PD-L1 blocking antibody increased the expression of granzyme-B and perforin on polyclonal- and tumor-stimulated CD8+ T cells. Taken together, our data show that rather than being exhausted, tumor-responding CD8+ T cells are not completely differentiated into effector cells and are prone to negative regulation by PD-L1. Hence, our study provides evidence that lung cancer patients respond to immunotherapy due to blockade of the PD-L1/PD-1 pathway.